FIG. 1 shows an apparatus, disclosed in U.S. patent application Ser. No. 08/384,997, for electrically suppressing the electrochemical potential (ECP) near a BWR component which is susceptible to intergranular stress corrosion cracking (IGSCC). The apparatus is a self-contained means of locally protecting critical portions of metals, such as welds, by suppressing ECP in the immediate vicinity of that portion of the metal requiring protection in operating BWR plants.
The apparatus shown in FIG. 1 is based on the concept of supplying electrons directly and locally to the surface of a sensitized metallic structural member 2, as in the case of the heat affected zone 6 of a weld 4, thereby inhibiting IGSCC. The electrical system depicted in FIG. 1 is capable of supplying sufficient electrons to the metal surface to inhibit the corrosion reaction due to local ECP exceeding the threshold value at which IGSCC can occur.
In the circuit of FIG. 1, the center electrical conductor of a small mineral-insulated steel sheathed cable 16 is attached to the metal surface to be protected against IGSCC and connected to an electrical control circuit 10 that operates off the low-voltage DC power supply 20. The control circuit 10 and DC power supply 20 are enclosed in a housing 8 made of material able to withstand thermal and radiological conditions inside a boiling water reactor, but outside the reactor core. The passive conductor of a twisted-shielded pair of cable conductors is connected to a reference electrode 18 located in the oxidizing coolant near the metal surface and to a terminal of the control circuit. The current collected at the metal surface is controlled by the applied voltage on the load resistor R via an electrical conductor connected to the surface of the metal to be protected and to another terminal of the control circuit. This current I is converted to a voltage drop across R, which is input to a differential amplifier 12 of gain G. The differential amplifier output is the effective voltage "error signal", which is integrated by the operational amplifier 14 with time constant .tau.=R.sub.1 C. The small stand-off resistor R.sub.2 depletes excess charge build-up on the feedback capacitor C to eliminate any possibility of integrator malfunction. The collected current is dissipated in the load resistor R. Electron depletion of the metal and IGSCC are defeated since electrons are forced to flow into the metal to compensate for those that would be lost by oxidation of the metal.
The apparatus shown in FIG. 1 has a power supply 20 which requires no external power source, but rather is energized by electrons (also referred to herein as .beta.-particles) produced during nuclear decay. The source of electrons was the .beta.-decay of a radioactive isotope, depicted in FIG. 1 as a current source 22.
.beta.-decay is a common decay mode of many nuclear isotopes whereby the nucleus spontaneously converts a neutron to an energetic electron (.beta.-particle) ejected from the nucleus, a proton retained in the nucleus and an anti-neutrino. .beta.-decay is a manifestation of the so-called weak force in the nucleus, which obeys all the classical conservation laws, except parity in a small percentage of decay events. It is known, both theoretically and experimentally, that the emitted electron cannot exist in the nucleus prior to emission. It is created, in every respect exactly like any electron, during the decay process, in which the original nucleus is converted to a new element with the same mass number (A), but with one additional proton (Z.fwdarw.Z+1).
.beta.-particles (or rays) can carry substantial kinetic energy when emitted. The energy distribution (spectrum) of these particles is continuous end displays a maximum energy, above which no particles exist. Their absorption in materials is known to vary inversely as a power law in the maximum energy (E.sub.max) and the spectrum is unchanged by absorption. In particular, the measurable activity of a .beta.-emitter is limited by self-absorption in the source itself, a factor that must be considered when designing devices employing .beta.-radiation. Typically .beta.-particles can be stopped by a millimeter thickness of most materials. Therefore, .beta.-sources are inherently weak sources of radiation dosage.
Alternative modes of decay usually exist for many .beta.-emitters, such as electron capture, internal conversion, isomeric transition (.gamma.-emission), and positron emission. These competing modes do not produce useful electrons, so only the fraction of decay events that produce .beta.-rays are of interest in the current context. Self-absorption also limits useful decay events to those that have a substantial maximum .beta.-decay energy E.sub.max of the order of 1 MeV or greater. Therefore, all .beta.-emitters are not viable candidates for use in a .beta.-battery. In fact, it is not obvious that any isotope(s) exist with the requisite properties and lifetime.